1. Field of Invention
The invention relates to a method for operating a light source of a motor-vehicle headlamp using direct current while the headlamp was intended to be operated with alternating current. At the same time, the light source has an inductive load. The light source is wired between branches of an H-bridge circuit with four semiconductor switches. When operating with alternating current, the light source is supplied with the alternating current via the H-bridge circuit. In the process, both of the high-side semiconductor switches of the H-bridge circuit are extensively controlled, respectively, by at least one bootstrap capacitor.
Also, the invention relates to an electric circuit for operating a light source of a motor-vehicle headlamp using direct current while the headlamp was intended to be operated with alternating current, wherein the light source has an inductive load. The circuit includes an electric H-bridge circuit with four semiconductor switches for supplying the light source in the “AC” operation with alternating current (wherein the light source is wired between branches of the H-bridge circuit) and bootstrap circuits each provided with a bootstrap capacitor (wherein each of the bootstrap circuits is controlling one of the two high-side semiconductor switches of the H-bridge circuit).
Furthermore, the invention relates to a light module of a motor-vehicle headlamp. The light module includes a light source with inductive load and a circuit for operating the light source.
Finally, the invention relates to a motor-vehicle headlamp that includes a housing having a light-emitting aperture that is closed with a transparent cover plate and at least one light module arranged in the housing.
2. Description of Related Art
For example, a gas-discharge lamp (GDL) is a light source with an inductive load. The inductive load results from the inductive properties of a high-voltage ignition transformer for igniting and operating the gas-discharge lamp. Gas-discharge lamps are used in light modules of motor-vehicle headlamps.
The light module with the gas-discharge lamp can be designed as a reflection module in which the light emitted from the gas-discharge lamp is reflected by a reflector to generate a desired light distribution on the road in front of the motor vehicle. To generate minor variations in light distribution, it is possible to provide in the optical path optically effective profiles (for example, in the form of cylindrical lenses or prisms). The optical profiles can be designed on a cover plate (diffusion disc) of the motor-vehicle headlamp through which the light reflected by the reflector passes when it is emitted from the motor-vehicle headlamp. If the generated light distribution should feature a dimmed-light distribution with a basically horizontal cut-off limit, the shape of the reflector can be selected such that the reflected light emitted by the headlamps already has the desired cut-off limit without requiring additional means for switching off portions of the reflected light beams. Reflectors designed in this way are also called “free-form reflectors.”
Alternatively, the light module with the gas-discharge lamp can be designed also as a so-called “projection module” in which the light emitted by the gas-discharge lamp is first concentrated by a primary optical device (for example, a reflector) and then represented by a secondary optical device (for example, a projection lens) for generating a desired light distribution on the road in front of the motor vehicle. If the generated light distribution should feature a basically horizontal cut-off limit (for example, for generating a dimmed-light distribution, like low-beam light or fog light) and/or a vertical cut-off limit [for example, to realize partial high-beam light (other road users in front of the motor vehicle are specifically shielded from high-beam-light distribution) or a marker light (objects or persons in front of the motor vehicle are specifically illuminated)], at least a respective shutter arrangement can be arranged in the optical path between the primary and the secondary optical device, which shutter arrangement blocks a portion of the concentrated light beams. A basically horizontal upper edge (for generating the horizontal cut-off limit) and/or a basically vertical lateral edge (for generating the vertical cut-off limit) of the shutter arrangement is represented by the secondary optical device as “light/dark” transition on the road in front of the motor vehicle.
To vary the light distribution generated by the projection module, the shutter arrangement can be inserted to a greater or lesser extent in the optical path. In this way, the generated light distribution can be switched (for example, between low-beam light and high-beam light) or the position and/or extent of a blocked-off area (when using partial high-beam light) or an illuminated area (when using marker light) can be changed. Furthermore, to achieve a variation in light distribution, it is possible to vary the progression of the represented edges of the shutter arrangement. Moreover, it is possible to select further light distributions (for example, between low-beam light and high-beam light). The light distributions are defined by intermediate positions of the shutter arrangement between the position for low-beam light and the position for high-beam light and/or by varying the progression of the edge of the shutter arrangement represented by the secondary optical device. For example, such further light distributions involve city-light distribution, highway-light distribution, expressway-light distribution or rain, snow, or other bad-weather light distribution or the like.
Usually, gas-discharge lamps are operated with alternating current to avoid, among other things, overheating of the electrodes between which the electric arc is generated and to avoid associated consequences, including a failure of the gas-discharge lamp. Gas-discharge lamps can be dimmed only to a limited extent (i.e., operated with reduced current because with lower currents the electrodes cool off, which increases the “electron work” function, but reduces the light-emitting ability). The lower the current, the more re-ignition voltage has to be applied for the reversal in current direction when operating with alternating current. Low currents or outputs result in commutation problems that appear as light flickering or can result even in completely extinguishing the electric arc.
It is possible to design a control circuit for gas-discharge lamps such that the gas-discharge lamp can be dimmed when operated with alternating current. To prevent the electrodes from cooling off too much and avoid the associated commutation problems, the control circuit could just before switching increase the output by, for example, increasing the current from the usual value of, for example, 600 mA required for operation to, for example, approximately 1 A. This involves an output increase of approximately 50%. As a result, the electrodes could be heated prior to switching, reducing the work function and the voltage required for switching. However, this is associated with higher energy consumption and higher stress on the control circuit and the gas-discharge lamp and reduced service life because of the intermittent process of heating the electrodes.
Because of their special structure, gas-discharge lamps of more modern design provide the possibility starting at a specific non-critical electrode temperature to switch from “alternating current” operation to “direct current” operation without damaging the lamp. At the same time, the electrodes of the gas-discharge lamp are designed such that, during “direct current” operation, a thermal balance occurs on an electrode that, on the one hand, incorporates the temperature increase resulting from the electrons/ions arriving at the electrode and, on the other hand, a temperature reduction resulting from the heat dissipation taking place via the material of the electrode. “Direct current” operations allow for considerably lower outputs without causing the above-mentioned light flickering or even completely extinguishing the electric arc as a result of commutation problems.
To operate gas-discharge lamps with alternating current, usually a so-called “H-bridge circuit” is used. The Hl-bridge circuit includes four controllable semiconductor switching elements (in an embodiment, in the form of transistors) and converts DC-link direct voltage into square-wave alternating current by converting one branch of the H-bridge circuit in which one side of the load is arranged with a specific frequency between the positive-supply voltage and ground, on the one hand, and the other branch of the bridge in which the other side of the load is arranged between ground and the positive-supply voltage, on the other hand. In the process, the voltage over the load is represented as differential voltage of the branches of the bridge that switch back and forth between positive and negative DC-link voltage. In the circuits known from the prior art, depending on the design of the semiconductor circuit and its control electronics, a permanent “direct current” operation can be maintained only with more or less extensive effort.
When using N-channel MOSFETs or IGBTs as high-side switches in which the drain is connected with the positive DC-link voltage V+ (for example, V+=500V), the gate potential for control has to be higher than the source potential at least by the threshold voltage Uth (for example, Uth=2 to 4 V). In switched-on state (when the semiconductor switch is conductive), drain and source are almost on the same potential. Therefore, the gate has to be provided with a voltage that is at least by the threshold voltage higher than the DC-link voltage V+ (for example, Vg=510 V). In the prior art, this is usually realized with a so-called “bootstrap circuit.” The bootstrap circuit includes a gate driver that is controlled via a level shifter and the supply voltage of which consists of a charged bootstrap capacitor having a base point at the source of the high-side switch (corresponding to the drain of the low-side switch and the output Uout of the H-bridge circuit), and an opposite pole is applied via a high-voltage bootstrap diode at a supply voltage Vs (for example, Vs=10 to 12 V). When controlling the low-side switch, the output Uout of the H-bridge circuit is applied to ground, and the bootstrap capacitor can be charged via the bootstrap diode approximately to supply voltage Vs. When the bridge circuit should be converted, the low-side switch is switched off (interrupted), and subsequently the high-side switch is controlled (conductive). As a result, the load from the bootstrap capacitor is switched via the gate driver between gate and source of the high-side switch, whereby the potential of the source (corresponding to the output voltage Uout of the H-bridge switch) to the DC-link voltage V+. As a result, the entire bootstrap circuit in the potential is shifted to the DC-link voltage V+, and the bootstrap diode is blocked, wherein the gate voltage of the high-side switch is higher than the DC-link voltage V+ (approximately 500 V) by the bootstrap voltage (approximately 10 to 12 V).
Through stray currents (supply of the gate driver plus reverse current of the bootstrap diode plus reverse current of the level shifter), the bootstrap capacitor is discharged, whereby the high-side switch involuntary and independently is switched off when it falls below the threshold voltage Uth. When operating the gas-discharge lamp with alternating current, this does not cause any problems. However, it considerably restricts the period of time in which the gas-discharge lamp can be operated in “DC” operation. The maximum time in which the high-side switch can continue to operate is defined by the capacity of the bootstrap capacitor, the stray currents, the supply voltage Vs, and the threshold voltage Uth of the semiconductor voltage. However, the maximum period of time is definitely restricted, which determines the minimum operating frequency of “AC” operation. Consequently, by the control circuit described above and known from the prior art, it is not possible to perform a longer or even unlimited “DC” operation of the gas-discharge lamp.
However, to make “DC” operation of a gas-discharge lamp possible, the load discharged from the bootstrap capacitor has to be tracked. According to the prior art, this is realized by using complex high-voltage pump circuits (level shifter with capacitive charge pumps or potential-free transformer coils with rectifier). For example, DE 11 2007 000 465 T2 discloses that it is necessary to use additional transformer coils of an already available low-voltage transformer, an additional high-voltage diode (designed for up to 600 V) with particularly small stray currents, and an additional Zener diode connected in parallel to the bootstrap capacitor to be able to track the load flowing from the bootstrap capacitor. By these additional circuit elements, the bootstrap capacitor can be charged with temporary current pulses during “DC” operation and, as a result, track the load flowing from the bootstrap capacitor. However, the proposed solution has the disadvantage that it results in considerably higher energy consumption and requires additional components. Therefore, in DE 11 2007 465 T2, the required additional components are provided only for a bootstrap circuit. This has the disadvantage that “DC” operation is possible only in one direction. Furthermore, the transformer coils are positioned on the potential of the DC-link voltage V+ (approximately 500 V) and has to be elaborately insulated toward the low voltage of the low-voltage transformer.
Based on the prior art described above, the invention has the objective of providing dimming or power control of gas-discharge lamps in “DC” operation despite using bootstrap-control without requiring additional measures or components.